DENEBOLA (Beta Leonis). Great Leo, who
seems to rule northern spring skies, contains three stars of major
note: bright Regulus, second magnitude
Algieba (which shares the "Sickle" with Regulus), and mid-second magnitude (2.14)
Denebola, the 62nd brightest star in the sky. Denebola, Leo's Beta
star (though marginally third brightest after binary Algieba), is the
easternmost of a prominent triangle of stars set to the east of
Regulus. It provides us with the Lion's tail, the name coming from
the Arabic phrase, "dhanab al-asad," which means exactly that.
Denebola, a white class A (A3) hydrogen-fusing dwarf star with a
temperature of 8750 degrees, shines to us from a relatively nearby
distance of 35.9 light years, give or take a mere 0.2 ly (second
Hipparcos reduction). Like all the brighter naked eye stars,
Denebola is more luminous than the Sun,
emitting 13.8 times the solar energy, from which we find a radius
of 1.62 times solar. Direct interferometric measure of angular
diameter combined with the distance gives 1.65 solar radii, which
is consistent with the temperature (no surprise since the
temperature was derived from the angular size). A rather rapid
projected equatorial rotation speed of 125 kilometers per second
yields a rotation period under 0.65 days, the speed enough to
prevent chemical peculiarities (by separation in a quiet
atmosphere), the iron content also close to the solar value.
Denebola is a prominent member of the fairly common "Vega" class of stars that are surrounded by
disks of infrared-emitting
dust. Since the planets of our Solar System were apparently
created from such a circumstellar dusty cloud, Denebola's dust
implies the possibility that the star might have planets as well, though there is no direct
evidence for them. But direct infrared observation of the dust
cloud (N. D. Stock et al., Astrophysical Journal, vol. 724, p.
1238, 2010) reveals a "complex structure" with a hole in the middle
an Astronomical Unit in radius, a "dusty ring" 2 to 3 AU in radius
heated to about 600 Kelvin, and a 120-Kelvin "broad dusty emission
zone extending from about 5 AU to 55 AU." A commonly-seen
"dominant outer belt near 100 AU" is missing, the meaning of its
lack unknown. Denebola is listed as a subtle variable star of the
"Delta Scuti" type. Such stars vary in
brightness by small amounts over periods of only hours. However,
detailed observations show that it is a spurious designation, the
star not varying significantly at all. Three faint "companions"
(16th magnitude "B" 40 seconds of arc away from an observation in
1898), 13th magnitude "C" 99 seconds distant, and 9th magnitude "D"
some minutes off, all seem to be line-of-sight coincidences. "B"
is problematic. It seems to have "disappeared" and may be spurious
or might even have been recovered as "C." (Thanks to Bill Hartkopf
and Brian Mason.)